A Low Noise Amplifier (LNA) is a device used as a front end to amplify signals in several applications, for example radio frequency receivers. A common-source architecture of the LNA provides low noise as compared to a common-gate architecture. However, the common-source architecture requires additional components for impedance and noise matching which in turn increases area.
An exemplary LNA 100 with a common-gate architecture is illustrated in FIG. 1 (Prior Art). An amplifier 105 and a device 110 are coupled in a cascode connection, and provide an output at a drain of the device 110. A source of the amplifier 105 is coupled to an input terminal 155 that is connected to a signal source or antenna. An equivalent of the signal source or the antenna is shown as a resistor 160. Impedance matching is performed by an impedance matching circuit 140 that includes an inductor 145 and a capacitor 150 in a parallel connection. The impedance matching circuit 140 matches an impedance of a load 120 that includes a parallel connection of an inductor 125 and a capacitor 130 with impedance present at the input terminal 155. A noise for the LNA 100 is presented in equation (1).Fo=1+(in2/is2)*1/(gmRs)2  (1)where Fo is a reference noise factor, in2 is an equivalent noise of the amplifier 105 and the device 110, is2 is an equivalent noise of the resistor 160, gm is a transconductance of the amplifier 105, and Rs is an equivalent resistance of the signal source or the antenna.
From equation (1), the noise for the LNA 100 with the common-gate architecture is found to be greater than that of an LNA with the common-source architecture. Hence, it is desired to improve noise performance of the LNA, for example the LNA 100 having the common-gate architecture.